1. The Field of the Invention
The present invention relates generally to a buoyancy module or can for supporting a riser of a deep water, floating oil platform. More particularly, the present invention relates to a buoyancy module with a tapering wall thickness.
2. The Background Art
As the cost of oil increases and/or the supply of readily accessible oil reserves are depleted, less productive or more distant oil reserves are targeted, and oil producers are pushed to greater extremes to extract oil from the less productive oil reserves, or to reach the more distant oil reserves. Such distant oil reserves may be located below the oceans, and oil producers have developed offshore drilling platforms in an effort to extend their reach to these oil reserves.
In addition, some oil reserves are located farther offshore, and hundreds of feet below the surface of the oceans. Floating oil platforms, known as spars, or Deep Draft Caisson Vessels (DDCV) have been developed to reach these oil reserves, examples of which are described in U.S. Pat. Nos. 4,702,321 and 5,558,467. Steel tubes or pipes, known as risers, are suspended from these floating platforms, and extend the hundreds of feet to reach the ocean floor, and the oil reserves.
It will be appreciated that these risers, formed of hundreds of feet of steel pipe, have a substantial weight which must be supported by buoyant elements at the top of the risers. Steel air cans have been developed which are coupled to the risers and disposed in the water to help buoy the risers, and eliminate the strain on the floating platform, or associated rigging. One disadvantage with the air cans is that they are formed of metal, and thus add considerable weight themselves. Thus, the metal air cans must support the weight of the risers and themselves. In addition, the air cans are often built to pressure vessel specifications, and are thus costly and time consuming to manufacture.
In addition, as risers have become longer by going deeper, their weight has increased substantially. One solution to this problem has been to simply add additional air cans to the riser so that several air cans are attached in series. It will be appreciated that the diameter of the air cans is limited to the width of the platform structure, while the length is merely limited by the practicality of handling the air cans. For example, the length of the air cans is limited by the ability or height of the crane that must lift and position the air can. One disadvantage with more and/or larger air cans is that the additional cans or larger size adds more and more weight which also be supported by the air cans, decreasing the air can""s ability to support the risers. Another disadvantage with merely stringing a number air cans is that long strings of air cans may present structural problems themselves. For example, a number of air cans pushing upwards on one another, or on a stem pipe, may cause the cans or stem pipe to buckle.
It has been recognized that it would be advantageous to optimize the systems and processes of accessing distant oil reserves, such as deep water oil reserves. In addition, it has been recognized that it would be advantageous to develop a system for reducing the weight of air cans, and thus the riser system and platforms. In addition, it has been recognized that it would be advantageous to develop a system for increasing the buoyancy of the air cans.
The invention provides a buoyancy system and or buoyancy module. The buoyancy module is vertically oriented, disposed below the surface of the water and coupled to a riser or stem pipe to support the riser. One or more buoyancy modules may be sized to have a volume to produce a buoyancy force at least as great as the riser.
The buoyancy module includes an elongate vessel with a vessel wall, and upper and lower ends. The vessel wall has a thickness that advantageously varies from a thinner wall thickness at the lower end, to a thicker wall thickness at the upper end. The upper end with the thicker wall thickness is disposed at a lower water pressure, and the lower end with the thinner wall thickness is disposed at a higher water pressure. The vessel may be internally pressurized with air so that an internal air pressure of the vessel substantially equals the higher water pressure at the lower end of the vessel. Thus, a lower pressure differential exists at the lower end with the thinner wall thickness, and a higher pressure differential exists at the top end with the thicker wall thickness.
In accordance with one aspect of the present invention, the vessel wall tapers substantially continuously. Alternatively, the vessel wall may include at least two different sections of continuous or constant thickness. A lower section may have a thinner continual thickness, and an upper section may have a thicker continual thickness.
In accordance another aspect of the present invention, the riser may be over 1000 feet long with an associated weight, and the buoyancy module advantageously may include an elongated vessel with a composite vessel wall. Preferably, the composite vessel wall advantageously has a decrease in weight when submerged. In addition, the composite vessel wall preferably has a density less than the density of the riser. Furthermore, the composite vessel wall preferably has a coefficient of thermal expansion less than a coefficient of thermal expansion of the riser. The composite vessel wall also may have a thermal conductivity less than a thermal conductivity of the riser.
In accordance with another aspect of the present invention, the buoyancy module may include a stem pipe which extends concentrically within the vessel, with an upper end of the vessel coupled to the stem pipe. The riser is received through the stem pipe. Alternatively, the buoyancy vessel or module may be coupled directly to the riser.
A spider structure may be attached to the vessel to position the stem pipe concentrically within the vessel. The spider structure may have an annular member with an aperture receiving the stem pipe therethrough, and a plurality of arms attached to and extending between the vessel and the annular member.
In accordance with another aspect of the present invention, more than one buoyancy modules advantageously may be limited to manageable sized but coupled together to achieve a desired buoyancy. A second elongate vessel may have an upper end directly attached to the lower end of the first elongate vessel. The first and second elongate vessels may have different lengths, and different volumes.
Additional features and advantages of the invention will be set forth in the detailed description which follows, taken in conjunction with the accompanying drawing, which together illustrate by way of example, the features of the invention.